Swept source optical coherence tomography (OCT) method and system

ABSTRACT

A method and apparatus are provided for a swept source optical coherence tomography (OCT) system utilizing a fast scanning mechanism in the sample arm and a slowly swept light source. The position data is collected rapidly while the wavelength of the source is swept slowly. The system reduces the sweep speed requirements of the light source enabling higher power, greater imaging range, and linear sweeps of the source frequency. The OCT components (or most of them) may be implemented within a hand held imaging probe. In operation, a triangulation scan may be used to orient the imaging probe with respect to a fixed coordinate system; preferably, OCT data captured by the device is then transformed to that same orientation with respect to the fixed coordinate system to improve the scanning results.

This application is based on and claims priority from Ser. No.61/170,886, filed Apr. 20, 2009, Ser. No. 61/172,513, filed Apr. 24,2009, and Ser. No. 61/173,714, filed Apr. 29, 2009.

BACKGROUND

Optical coherence tomography (OCT) is a depth-resolved imaging modality.Reflections of light returning from within the tissue are used to createtomograms of the tissue microstructure.

A known OCT system is described in U.S. Pat. No. 7,355,721. The OCTimaging system described there includes a light source, an opticalcoupler or beam splitter, a reference arm, a projector, and a sensor.The OCT imaging system also may be coupled to a processor.

An alternative OCT technique uses a swept source. In one knownimplementation, the wavelength or frequency of a laser is swept over arange supported by the laser's gain medium. This form of OCT is calledswept source OCT or optical coherence domain reflectometry (OCDR).

A standard swept source OCT system is illustrated in FIG. 1. The source100 emits light within the visible and infrared regions of theelectromagnetic spectrum. At any instant of time, a laser with anarrowband of wavelengths is emitted from the light source. The sourceuses a spectral filter within the cavity to control the laserwavelength. The range of emitted wavelengths is dependent on a gainmedium of the source. An exemplary swept source emits a laser with aninstantaneous line width of 0.1 nm that is swept from 1250 to 1350 nm.

A circulator 102 directs light from the swept source to a 2×2 coupler104. The 2×2 coupler splits and directs a portion of the light to thereference and sample arms (106, 108) of a Michelson interferometer. Thereference arm 106 provides a fixed optical path length. The sample arm108 has approximately the same optical path length as the reference arm106. The sample arm includes optical and scanning elements necessary tofocus and position the beam into tissue. Light reflecting from the twoarms are combined at the 2×2 coupler. The two beams containing theinterfering signals are sent to a dual balanced detector 110. The datais then acquired and processed using a computer (not shown). Thisprocessing may include re-sampling of the waveform and Fouriertransformation.

In the prior art, the sweep rate of the light source governs the imageacquisition rate of the OCT system. Each sweep of the source correspondsto one column, that is, an axial scan, through the tissue. A slowerlateral scan within the sample arm is used to collect multiple axialscans for a 2D image. This process is illustrated in FIG. 2. At oneposition of a scan mirror 200, a data acquisition system (not shown)acquires the interference signal as a function of the wavelength (orfrequency) emitted by the swept source. The process occurs for eachsubsequent position of the scan mirror until enough axial scans arerecorded for a 2D image.

As shown in FIG. 3, the swept source OCT system produces a twodimensional matrix 300 with wavelength represented in the 1^(st)dimension and lateral position in the 2^(nd) dimension. In this systemof the prior art, this matrix 300 is populated by acquiring each column302 sequentially. A computer is often used to resample and Fouriertransform each column of the data matrix.

In the prior art OCT system, a greater sweep rate is required from theswept laser to image with greater speed. Sweeping a laser at a fastrate, however, often comes with negative consequences. When the sweeprate increases, the light travels less through the gain medium resultingin a decrease in the optical power emitted by the source. Theinstantaneous line width of the source can be increased to provide moreoptical power, but this reduces the useful imaging range of the OCTinstrument.

The subject matter of this disclosure addresses these and otherdeficiencies of the prior art.

BRIEF SUMMARY

A method and apparatus are provided for a swept source optical coherencetomography (OCT) system utilizing a fast scanning mechanism in thesample arm and a slowly swept light source. The position data iscollected rapidly while the wavelength of the source is swept slowly.The system reduces the sweep speed requirements of the light source,enabling higher power, greater imaging range, and linear sweeps of thesource frequency.

According to another feature of the disclosed subject matter, thesample, reference, and detection arms of an OCT interferometer areincorporated into an imaging probe, preferably using freespace opticalelements.

According to still another feature, preferably a triangulation scan isused to orient the imaging probe device with respect to a fixedcoordinate system, and OCT data is transformed to that same orientationwith respect to the fixed coordinate system. This approach enables adental imaging system to provide 3D digitizing using OCT withtriangulation-based registration of multiple scans, preferably from ahand held device.

Other features and advantages of the invention will be apparent to onewith skill in the art upon examination of the following figures anddetailed description. It is intended that all such additional featuresand advantages be included within this description, be within the scopeof the invention, and be protected by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

FIG. 1 illustrates a known swept source optical coherence tomography(“OCT”) imaging system.

FIG. 2 illustrates the sample arm optics and imaging technique used inthe OCT imaging system of FIG. 1.

FIG. 3 illustrates a two (2) dimensional matrix generated using theknown swept source OCT imaging system.

FIG. 4 illustrates a two (2) dimensional matrix generated usingaccording to the teachings of this disclosure.

FIG. 5 illustrates an imaging probe that comprises a set of OCT opticalelements that implement a swept source scanning technique.

DISCLOSURE OF AN EMBODIMENT

In this disclosure, in contrast, multiple lateral scans over the samearea preferably are performed during a wavelength sweep of the lightsource. Preferably, the lateral scan is performed with resonant scanningmirrors or polygonal scanning mirrors. In this arrangement, the OCTsystem becomes limited only by the lateral scan speed of the sample armrather than the sweep speed of the light source. Preferably, multiplelateral scans are performed over the same region. During each lateralscan, the wavelength of the source remains nearly constant. Thewavelength of the source changes for each subsequent lateral scan. Foreach position along the lateral scan, the interference signal isrecorded versus the wavelength or frequency of the swept source.

In the prior art, a laser sweep rate typically is a product of a framerate (images/second) and a number of axial scans per image. A sweep rateof this light source herein preferably is the laser scan rate on thespecimen divided by a number of wavelength samples. The approachdescribed herein permits the same imaging frame rates with a laser sweeprate several orders of magnitude slower as compared to the prior art.For example, assume that 1000 axial scans are needed for an image andthat each axial scan consists of 1000 samples at different wavelengths.To acquire the image at 60 frames per second, the prior art light sourcewould need to sweep through the gain medium's wavelength range at 60 kHz(1000×60). In the approach herein, however, the light source sweepsthrough its wavelength range at 60 Hz.

FIG. 4 demonstrates the difference between this disclosure and the priorart with the construction of the data matrix. According to the techniquedescribed in this disclosure, a row 402 of the matrix 400 is populatedduring one lateral scan in the sample arm. Each subsequent lateral scanoccurs at a different wavelength (or frequency) until the entire sweepof the source is complete. The data is the same as the prior art and thesame processing occurs, however, the data acquisition method isdistinctly different and provides significant advantages.

In particular, this technique provides several key advantages over theprior art. As noted above, for a given frame rate and image size, thispresent approach enables slower wavelength sweep speeds as compared tothe prior art. The slower sweep speeds as contemplated herein allow thelight to travel many times through the gain media. This maximizes theamount of light amplification, which results in greater opticalintensities emitted from the source. Further, as the light has time tomake many passes through the gain media, the instantaneous line width ofthe spectral filter within the source cavity can be reduced. Thisreduced instantaneous line width results in greater imaging range of theswept source.

To provide high sweep rates, swept source technologies generally havenon-linear scans through optical frequency. The “slower” sweep rates ofthe disclosed technique permit linear sweeps with wavelength or opticalfrequency. Linear sweeps can simplify signal re-sampling techniquesassociated with prior art swept source OCT systems.

An illustrative commercial swept source that may be adapted to practicethe technique described above is the HSL-2100 from Santec Corporation.Other alternatives include, without limitation, the SS225 from MicronOptics, and the SL1325-P16 from Thorlabs. These are merelyrepresentative.

The subject disclosure may be implemented using base OCT technologies,such as the optical elements and related processing devices andsub-systems such as described in U.S. Pat. No. 7,355,721, the disclosureof which is incorporated herein by reference.

In addition, the disclosed subject matter may be implemented isconjunction with a computer workstation that comprises hardware,suitable storage and memory for storing an operating system, one or moresoftware applications and data, conventional input and output devices (adisplay, a keyboard, a mouse, etc.), other devices to provide networkconnectivity, and the like. Optical elements such as described in U.S.Pat. No. 7,184,150 may be implemented as needed.

FIG. 5 illustrates another aspect of the disclosed invention. In thisembodiment, a hand held imaging probe 500 is connectable to an OCT lightsource 502 through an optical fiber 504, and the imaging probe is usedto capture the OCT data. In this approach, and as can be seen in FIG. 5,preferably the sample, reference, and detection arms of the OCTinterferometer are incorporated into the imaging probe using freespaceoptics. Representative freespace optics are available commercially fromvarious suppliers including, without limitation, Thorlabs, Newport, andEdmund Optics.

Although not a limitation of this disclosure, preferably a triangulationscan is used to orient the imaging probe device of FIG. 5 with respectto a fixed coordinate system, and the OCT data collected by that deviceis transformed to that same orientation with respect to the fixedcoordinate system. Thus, using the device shown in FIG. 5, a dentalimaging system may provide 3D digitizing using OCT withtriangulation-based registration of multiple scans.

To this end, the imaging probe comprises sample arm 506, reference arm508, laser source 510, dichroic mirror 512, beam splitter 514, scanningmirrors 516 and detector 518. Inside the imaging probe, the light iscollimated and then split into the reference and sample arms of the OCTsystem. This is performed by either a freespace beamsplitter 514 or afiber optic 2×2 coupler. Light in the reference arm is directed to amirror using a combination of prisms and lenses (not shown). The mirroris positioned such that the optical path length of the reference arm isapproximately equal to the sample arm path length. The beam of thesample arm passes through the dichroic mirror 512. As noted above,preferably this device combines the OCT light and laser triangulation(LT) light into one beam. The combined OCT/LT beam is reflected by oneor more scanning mirrors 516. These mirrors enable the beam to bescanned across the surface of the tissue. The scanned beam istransmitted to the distal end of the probe using a lens assembly 520.The lens assembly is designed to accommodate the scan angles anddifferent wavelength bands of the OCT and LT subsystems. The lenses arealso used to focus the beam near the tissue surface. A prism deflectsthe beam towards the tissue.

In general, the scanning technique combines both methods described aboveinto a single device (such as shown in FIG. 5) and describes the meansto combine the various types of 3D data into a single unified data set.

Using the combined device, the oral cavity of a patient may be scannedas follows:

In the first step, the device is configured to scan in triangulationmode. Using this mode, the patient's full mouth may be fully scannedwithin minutes using the high speed triangulation method.

When this operation has been completed, a 3D mesh S of all thedirectly-visible surfaces in the oral cavity is computed and displayedon a computer screen. Following this, the device is configured to bescanned in the OCT mode. Using this mode, the device is positioned toobtain additional volume data in the desired location of an area thatwas previously scanned. When triggered, the scanner then performs asingle detailed OCT scan of the region, which we shall refer to as C.Preferably, C is a volume of densities on a three dimensional grid whereeach grid location has a position coordinate with respect to the originof the camera. i.e., if C is a grid of nx by ny by nz values, then eachposition C [x,y,z] (where x=[1, . . . ,nx], y=[1, . . . ,ny], z=[1, . .. ,nz]) corresponds to a 4-tuple (a,b,c,d) where (a,b,c) is a positionof the grid point in space relative to the origin of the device, and dis the density at that location as imaged.

The scanner also performs a single triangulation-based scan in that sameposition, which we shall refer to as T. Preferably, T is a twodimensional grid of position coordinates. i.e., if T is a grid of nx byny values, then each position T[x,y] ((where x=[1, . . . ,nx], y=[1, . .. ,ny]) corresponds to a triple (a,b,c) which is the position of thegrid point in space relative to the origin of the device.

Note that the OCT scan obtains a three dimensional grid of densities atlocations in space, whereas the triangulation scan only obtains a twodimensional grid of locations in space. This is due to the fact that thetriangulation method is only able to see surfaces, whereas the OCTmethod is able to see below the surface.

It is then possible to compute a 4×4 matrix M which when applied to T,transforms the points T to correspond to the same equivalent points inthe mesh S as computed during the previous triangulation only scanningprocess. This is done using the same process that is used to registertriangulation scans together, in other words by using a method such asICP (iterative closest pair matching).

We further assume that the device has moved only minimally during thisprocess and so we may then use the matrix M obtained above to transformthe volume grid C into the same coordinate space as the mesh C.

When this process is repeated a number of times, we obtain a collectionof volume grids that are positioned spatially in their correctorientations relative to each other even if the OCT scans arenon-overlapping.

These multiple grids may then be combined into a single spatial grid(for example, where there is no data from any constituent scan C, thedensity may be indicated as zero). This approach allows us toeffectively use the OCT technique over a large volume using a muchsmaller OCT scanning volume by merging multiple OCT scans together.

Thus, and although not meant to be limiting, preferably a lasertriangulation contouring system is included in the imaging probe design.This technique addresses the problem of locating the OCT data whenimaging in a confined area. Preferably, laser triangulation provides 3Dregistration of the OCT data.

The above-described laser triangulation and OCT data capture techniqueis not limited for use with the hand held imaging probe of FIG. 5, asthe techniques may be implemented in other devices or systems.

Using the above-described technique, the user may scan areas of interestin the oral cavity using the superior scanning ability of the OCTtechnique, and have all the separate scans be registered correctly withrespect to each other. Such a database can then be used verybeneficially in dentistry. For example, such a database could be used tocatalog all the different materials present in the mouth of a patient,by tooth. Such materials could include different restoration materials(such as ceramics, composites, metals, etc). The database could be usedto locate caries both at the surface, or interproximally, or evenunderneath existing non-metal restorations. The database could be usedto feature tartar deposits, plaque deposits or perio pocket depths. Bycomparing databases over time (for example comparing the data base tothat of the same patient six months prior), it would also be possible totrack changes such as gum loss. By combining these two technologieswithin a single device (which enables their data to be registered toeach other), it is possible to include diagnostic information within the3D data set. The diagnostic information is obtained from the volumetricnature of the OCT-derived data as well as the ability of OCT to identifyinterfaces between materials as well as relative and absolute densitiesof materials on the surface and volumetrically. This enables thetechnology to provide a 3D diagnostic map, including but not limited to,caries detection (decay) , cancer detection, inflammation, gum disease,periodontal disease and other disease and injuries, as well as keeptrack of changes in the above over time, thereby assisting in thediagnosis and tracking the oral health of the patient.

The database could also be used to produce restorations, which havetraditional been designed using triangulation data only. For example, apreparation where the margin is subgingival can now be done with ease,because the margin will now be fully visible even if hidden from directsight by biological tissue as long as that tissue is no more than about5 mm thick. An example of such an application is the scanning of asubgingival implant abutment. Due to the surgical nature of the implantprocedure, it usually is not possible to digitize the implant abutmentfully when it has been inserted into the cavity. By utilizing atechnology than can scan surfaces through biological tissues and fluids,it will be possible to design CAD/CAM restorations to fit on implantabutments within a single visit.

While certain aspects or features of the disclosed subject matter havebeen described in the context of a computer-based method or process,this is not a limitation of the invention. Moreover, such computer-basedmethods may be implemented in an apparatus or system for performing thedescribed operations, or as an adjunct to other dental restorationequipment, devices or systems. This apparatus may be speciallyconstructed for the required purposes, or it may comprise a computerselectively activated or reconfigured by a computer program stored inthe computer (in which case it becomes a particular machine). Such acomputer program may be stored in a computer readable storage medium,such as any type of disk including optical disks, CD-ROMs, andmagnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), magnetic or optical cards, or any type of mediasuitable for storing electronic instructions, and each coupled to acomputer system bus. The described functionality may also be implementedin firmware, in an ASIC, or in any other known or developedprocessor-controlled device.

While the above describes a particular order of control operationsperformed by certain embodiments, it should be understood that suchorder is exemplary, as alternative embodiments may perform theoperations in a different order, combine certain operations, overlapcertain operations, or the like. References in the specification to agiven embodiment indicate that the embodiment described may include aparticular feature, structure, or characteristic, but every embodimentmay not necessarily include the particular feature, structure, orcharacteristic. Further, while given components of the system have beendescribed separately, one of ordinary skill will appreciate that some ofthe functions may be combined or shared in given systems, machines,devices, processes, instructions, program sequences, code portions, andthe like. Having now described our invention, what we claim is asfollows:

1. A method of optical coherence tomography (OCT) scanning using a lightsource, comprising: performing at least first and second lateral scansover an area to be imaged as a wavelength of the light source is swept;collecting data generated during each of the first and second lateralscans over the area; and generating a representation from the datacollected.
 2. The method as described in claim 1 wherein during a firstlateral scan the wavelength of the light source has a first,substantially constant value, wherein during a second lateral scan thewavelength of the light source has a second, substantially constantvalue, where the first and second values differ.
 3. The method asdescribed in claim 1 wherein each lateral scan is performed with aresonant scanning mirror or polygonal scanning mirror.
 4. The method asdescribed in claim 1 wherein the area is a structure located within anoral cavity.
 5. A swept source optical coherence tomography (OCT) systemcomprising a light source, an optical coupler, a reference arm, a samplearm, and a detector, comprising: a scanning mechanism associated withthe sample arm that generates two or more lateral scans over an area tobe imaged using two or more respective wavelengths of the light source;and a mechanism for collecting data generated by the scanning mechanism.6. The system as described in claim 5 wherein the light source is aslowly-swept light source.
 7. In an optical coherence domainreflectometry (OCDR) system having a light source, and a scanningmechanism in a sample arm, a method, comprising: performing multiplelateral scans over an area to be imaged as a wavelength of the lightsource is swept; and collecting data from the multiple lateral scans togenerate a representation.
 8. A probe constructed and adapted to be heldwithin a user's hand, comprising: an OCT interferometer comprising: alight source, an optical coupler, a reference arm, a sample arm, and adetector; and a scanning mechanism associated with the sample arm thatgenerates two or more lateral scans over an area to be imaged using twoor more respective wavelengths of the light source.
 9. The probe asdescribed in claim 8 wherein the scanning mechanism is operative toperform a scanning method, comprising: a. scanning an object in the areawith one or more triangulation scans to produce a mesh in a fixedcoordinate system; b. scanning the object at N further positionsoverlapping with the mesh, where at each scan location the object isscanned in a triangulation mode and in an OCT mode; c. computing atransformation matrix that registers the scan in triangulation mode ofstep (b) with the mesh obtained in step (a); d. applying thetransformation matrix to data collected from the scan in OCT mode togenerate transformed OCT data; and e. merging the transformed OCT datainto a mesh that is the same fixed coordinate system as described instep (a).